UV - Vis spectrophotometry is the determination of absorbance in 190 ~ 800nm wavelength range for identification, impurity testing and quantitative determination of the method. As light passes through the analyte solution, the degree of light absorption by the substance varies with the wavelength of the light. Therefore, by measuring the absorbance of the substance at different wavelengths and plotting its absorbance versus wavelength, the absorbance spectrum of the analyte is obtained. From the absorption spectrum, the maximum absorption wavelength λmax and the minimum absorption wavelength λmin can be determined. Absorption spectra of substances have structural features related to their structure. Thus, the material can be identified by a comparison of the spectra of the samples in a particular wavelength range with those of the control or reference spectra, or by determining the maximum absorption wavelength, or by measuring the absorption ratio at two specific wavelengths. For quantification, the absorbance of a sample solution at a certain concentration is measured at the wavelength of maximum absorption and compared with the absorbance of a control solution at a certain concentration or the absorption coefficient method is used to calculate the concentration of the sample solution.